1. Field of the Invention
The present invention relates to an optical head capable of reproduction of different kinds of information recording media, for example, reproduction of high-density optical disks (DVDs) and reproduction of compact disks (CDs).
2. Description of the Prior Art
In recent years, standards for a highly densified optical disk (hereinafter, referred to as DVD) have been announced and some of the manufactures have already commercialized the DVD. FIG. 9 shows the construction of a conventional optical system of an optical head for the DVD. In FIG. 9, a laser beam 2 emitted from a semiconductor laser 1 is reflected at a beam splitter 4 so that its direction is bent at the right angle, and is converted into a parallel beam 5 by a collimator 3. The parallel beam 5 is converged by an objective lens 7 and applied onto an information recording surface of an optical disk 8. The applied light is reflected at the optical disk 8 and is again incident on the objective lens 7, converted into the parallel beam 5 and converged by the collimator 3. The converged reflected light passes through the beam splitter 4 and is then detected by a photodetector 20.
According to the standards for the DVD, the thickness of the protection layer on the disk signal side is 0.6 mm, the wavelength of the signal reading laser is 655 nm and the NA (numerical aperture) of the reading objective lens is 0.6. These are greatly different from those of conventional CDs in which the thickness of the protection layer is 1.2 mm, the wavelength of the signal reading laser is 790 nm and the NA of the reading objective lens is 0.45. Therefore, in order to reproduce a conventional CD by use of an optical head for DVDs as shown in FIG. 9, some scheme is necessary. For this reason, various methods as described below have been proposed.
One method is to mechanically switch between an objective lens for the 1.2-mm-thick protection layer and an objective lens for the 0.6-mm-thick protection layer by use of some devices. Another method is to provide one objective lens with two focal points one for DVDs and the other for CDs like a bifocal lens. The method in which switching between two objective lenses is performed includes a method in which like an axially rotating head, two objective lenses of different kinds are disposed about a center of rotation and switching therebetween is performed by rotating them. The method in which one objective lens is provided with two focal points is an excellent method being simple in the construction of the optical system.
A still another method is to insert an aperture limitation element for both the light reaching the disk (go path) and the light reflected from the disk (return path). This method is intended to improve the quality of the CD reproduction signals, when a CD is reproduced by use of an objective lens for DVDs, by removing the light of a portion where the NA is great from both the light exiting from the objective lens and the light returning to the objective lens because reproduction signal deterioration is caused, particularly, in signals from the portion where the NA is great.
However, according to the method in which switching between two kinds of objective lenses is performed, the number of mechanical parts increases, so that the size of the optical head increases. In addition, the reliability is low because the switching between the lenses is performed by mechanical axial rotation of the lenses.
According to the method in which one objective lens is provided with two focal points, the use efficiency of the light is inferior because the light is inevitably divided into two parts. For this reason, the light quantity is insufficient for reproduction of low-reflectance RAM disks, etc. which will become important in the future.
According to the method in which the aperture limitation element is used, since aperture limitation is provided in a go and return system in which the light advances in both directions, device is necessary for removing the aperture limitation when a DVD is reproduced and inserting the aperture limitation when a CD is reproduced by some method; otherwise the aperture limitation is always inserted and as a natural consequence, it is impossible to reproduce a DVD. For this reason, the aperture limitation is formed of liquid crystal and by applying a voltage to the liquid crystal, the transmittance of the aperture limitation is changed to switch between reproduction of a DVD and reproduction of a CD. That is, the aperture limitation is equivalently removed and inserted. However, according to this method, it is necessary to provide more electric systems than before in the optical head optical system, which inevitably increases the size of the optical head.
As described above, while various methods have been considered to reproduce CDs by use of an objective lens (NA=0.6) for DVD reproduction, these conventional methods all use some scheme for both the go and return paths of the light. As mentioned previously, the heretofore contrived methods have problems such that the size of the apparatus increases, that the apparatus has a mechanical complexity and that DVD-RAM reproduction is impossible because the light quantity loss is great.